In a thermal printer, a dye donor member containing one or more dye colors is disposed between a receiver member, such as a sheet of paper, and a print head assembly formed of one or more thermal elements often referred to as thermal pixels. When a thermal pixel is energized to a certain level, the heat produced therefrom causes a dye color from the dye donor member to be transferred to the receiver member. The receiver member is mounted on an outside surface of a rotatable writing drum. The density (darkness) of the printed dye color is a function of the temperature applied by the thermal pixel to the dye donor member and the length of time the dye donor member is heated by the applied temperature (the energy delivered from the thermal pixel to the dye donor member). Thermal dye transfer printers offer the advantage of true "continuous tone" dye density transfer. This transfer is obtained by selectively varying the energy applied to each thermal pixel which results in a correspondingly selective variable dye density image pixel which is printed on the receiver member.
A first type of print head is formed with a plurality of resistive thermal elements forming the thermal pixels. The plurality of thermal pixels are usually organized into a plurality of groups of thermal pixels. The thermal pixels in each group are simultaneously addressed in parallel, and each group is addressed sequentially one at a time. In this manner, a smaller and less expensive power supply is needed than would be required when all of the thermal pixels are energized at the same time. In this regard see, for example, U.S. Pat. No. 4,621,271 (S. A. Brownstein, issued on Nov. 4, 1996) which describes method and apparatus for controlling a thermal printer arranged with a plurality of groups of thermal pixels. When a group of thermal pixels are addressed, the thermal pixels are each selectively energized and are driven by a constant voltage. More particularly, a technique is described which addresses the thermal pixels of each group a plurality of N times during a line printing period, and each of the thermal pixels of each group is selectively energized when the thermal pixel is addressed. In this manner each thermal pixel supplies thermal energy to the dye donor member which substantially corresponds to a desired dye color density to be reproduced in an image pixel on the receiver member.
A second type of thermal printer employs one or more laser beams which are each selectively energized as the beam impinges or scans each thermal pixel area on the surface of the dye donor member. The heat which is provided as the laser beam impinges the dye donor member in each pixel area determines the density level (amount of dye color transferred) on the receiver member in the pixel area. An exemplary thermal dye transfer printing apparatus using an array of semiconductor diode lasers is disclosed, for example, in U.S. Pat. No. 4,804,975 (K. Yip, issued on Feb. 14, 1989). Means are provided for controlling the laser diodes to produce light and selectively modulate the light from the individual lasers to provide sufficient energy to cause different amounts of dye to transfer from the dye donor member to the receiver member and form pixels with different levels of density.
With laser thermal printers, precise pixel resolution on the receiver member is required under certain conditions as, for example, for creating four-color proofs (defined in the graphic arts industry as the receiver member output from the thermal printer). In order to print dots (micropixels) at, for example, 1800 and 2400 dots per inch (dpi), it is desirable that the laser thermal printer maintain a small fractional part of micropixel resolution. When writing onto, for example, a receiver member wrapped about a writing drum, synchronization of pixel timing must repeat at regular intervals during the four-color proof printing. Slight changes in, for example, rotational speed or pixel timing has a cumulative effect in compromising the accuracy needed for creating the four-color image.
When graphical information, such as photographs or artwork, is printed in a typical publication, a half-tone printing process is used. Half-tone printing is capable of producing a very high fidelity reproduction of a photograph or artwork if various printing parameters are chosen correctly. Therefore, in four-color printing, half-tone dots of different colors have a precise positional relationship to each other. Because of such precise positional relationship between different colored half-tone dots, correct color registration is a requirement for half-tone color proofing. To create color proofs using laser thermal printing technology, a digital proofer (the laser thermal printer (LTP)) prints a single color at a time. Achieving accurate color registration means repeatable, precise positioning of the laser print head for each pass (for each color) during a proof writing process. Registration errors of only a small fraction of an inch can render a proof inaccurate thereby causing the proof to be colorimetrically incorrect or to have objectionable artifacts. Because the LTP printing process involves motion control in multiple axes (e.g., drum rotation and print head movement transverse to the drum rotation) and requires writing resolutions with typical values of, for example, 1800 and 2400 dpi, mechanical methods for achieving precise dot registration are prohibitively expensive and are prone to error due to timing and mechanical tolerance variations. Moreover, such mechanical methods do not allow writing speeds to be changed within a proof or between proofs. The problem is to provide a thermal printer capable of providing precise dot placement with each pass of the printing process.